1. Field of the Invention
The present invention relates to a motion transmission device, and more particularly to an extendible and retractable actuator.
2. Description of the Prior Art
The use of actuators are quite extensive, a motor of one kind of the actuators is mounted in an extendible and retractable sleeve and drives the sleeve to extend and retract, and then the extending and retracting motion of the sleeve drives other mechanisms to move.
Referring to FIG. 1, a conventional extendible and retractable actuator comprises a base A, an extendible and retractable sleeve assembly 10, a motor 20, a first screw 30, a second screw 40 and a third screw 50. The sleeve assembly 10 includes an outer sleeve 11, a middle sleeve 12 and an inner sleeve 13. One end of the outer sleeve 11 is fixed to the base A, and the motor 20 is fixed in the inner sleeve 13 of the sleeve assembly 10. The first screw 30 is a hollow member defined with outer threads on an outer surface thereof one end of the first screw 30 is connected to the motor 20 and is rotated by the motor 20, and the extending direction of the first screw 30 corresponds to the moving direction of the sleeve assembly 10. The second screw 40 is rotatably mounted in the middle sleeve 12 of the sleeve assembly 10, and the second screw 40 is a hollow member defined with inner threads to be engaged with the outer threads of the first screw 30. The third screw 50 is fixed to the base A and is defined with outer threads to be engaged with the inner threads of the second screw 40, and the third screw 50 is inserted into the first screw 30.
With reference to FIG. 2, during the operation of the extendible and retractable actuator, the motor 20 rotates the first screw 30 and drives it to move along the inner threads of the second screw 40, so as to drive the inner sleeve 13 to extend outward.
When the first screw 30 rotates to the maximum length position relative to the second screw 40 as shown in FIG. 3, the first screw 30 will be driven by the motor 20 to rotate the second screw 40, and the second screw 40 will move along the outer threads of the third screw 50 and drive the middle sleeve 12 to extend outward.
However, the above-mentioned conventional extendible and retractable actuator has the following disadvantages:
Firstly, the extending and retracting motion of the extendible and retractable actuator takes a lot of time, since the middle sleeve 12 can be driven to extend only when the inner sleeve 13 extends to the end of maximum travel length.
Secondly, the components of the conventional extendible and retractable actuator are likely to be damaged, since the second screw 40 can be driven to rotate only when the first screw 30 rotates to the end of maximum travel length.
Thirdly, the conventional extendible and retractable actuator is difficult to process, since the first screw 30 must be hollow and defined with outer threads, and the second screw 40 must be defined with inner threads to be engaged with the outer threads of the first and the third screws 30 and 50.
Therefore, another conventional extendible and retractable actuator appears on the market, as shown in FIG. 4, which comprises a base A, an extendible and retractable sleeve assembly 10, a motor 20, a first screw 30, a second screw 40 and a wheel 60. The sleeve assembly 10 includes an outer sleeve 11, a middle sleeve 12 and an inner sleeve 13. One end of the outer sleeve 11 is fixed to the base A, and the motor 20 is fixed to the base A. The first screw 30 is defined with outer threads, one end of the first screw 30 is connected to the motor 20 and is rotated by the motor 20, and the extending direction of the first screw 30 corresponds to the moving direction of the sleeve assembly 10. The second screw 40 is connected to the middle sleeve 12 and is inserted into the inner sleeve 13, and one end of the second screw 40 is open and is defined with inner threads to be engaged with the outer threads of the first screw 30. The wheel 60 is fixed to the other end of the second screw 40 and a rope 61 wind around it, one end of the rope 61 is fixed in the inner sleeve 13, and the other end of the rope 61 is fixed to the base A after passing through the inner sleeve 13 and the middle sleeve 12.
With reference to FIG. 5, during the operation of the extendible and retractable actuator, the motor 20 rotates the first screw 30 and drives it to move along the inner threads of the second screw 40, so as to drive the second screw 40 to move, and the second screw 40 drives the middle sleeve 12 to extend outward. At that time, the wheel 60 moves along the second screw 40 and drives the inner sleeve 13 to move, since one end of the rope 61 is fixed to the base A. Moreover, according to the principle of the movable pulley, the displacement of the inner sleeve 13 with respect to the outer sleeve 11 is double that of the displacement of the middle sleeve 12 relative to the outer sleeve 11. By such arrangements, the middle sleeve 12 and the inner sleeve 13 can be moved simultaneously, so as to reduce the extending and retracting time of the extendible and retractable actuator.
However, the above-mentioned conventional extendible and retractable actuator still has the following disadvantages:
Firstly, the above-mentioned conventional extendible and retractable actuator is hard to assemble, since the components are engaged with each other, and the middle sleeve 12 and the inner sleeve 13 can be moved simultaneously only when the rope 61 winds around the wheel 60.
Secondly, the components of the conventional extendible and retractable actuator are likely to be damaged, since the middle sleeve 12 and the inner sleeve 13 can be moved simultaneously only when the rope 61 winds around the wheel 60, the force applied on the rope 61 is quiet big, and the rope 61 will be broke if the force is too big. In addition, the rope 61 can be replaced by chain, however, the chain can also cause abrasion to the wheel 60.
The present invention has arisen to mitigate and/or obviate the afore-described disadvantages.